I tried to delete Main.java, and it won't let you. Fine, so I just erase the whole text file. Then, I place two separate folders w/ src code in them. Code compiles fine, but still prints out "Hello, world!".
Buggy. If you use Main.java, you can still have several packages and it all works, but I don't like that. What I might do is erase Main.java, then you're forced to manually run your program from their bash shell ($ java MyClass). And then it works fine.
I also don't like they're on openJDK 11. All the nice stuff is in 12-14 (text blocks, switch expressions).